Canning is a safe and economical way to preserve food quality at home. Canning home-grown food may save you significant money when compared to buying commercially canned food. Further, canning favorite and special products to be enjoyed by family and friends is often a fulfilling experience and a source of pride for many people. Moreover, if vegetables are handled properly and canned promptly after harvest, they can be more nutritious than fresh produce sold in local stores.
Because of the high percentage of water in most fresh foods, they are very perishable and spoil or lose quality because of the growth of undesirable microorganisms, such as bacteria, molds, and yeast; the activity of food enzymes; reactions with oxygen; and moisture loss. Microorganisms live and multiply quickly on the surfaces of fresh food and on the inside of bruised, insect-damaged, and diseased food.
Proper canning practices, however, can mitigate the perishable qualities of food. Proper canning practices include carefully selecting and washing fresh food; peeling some fresh foods; hot packing many foods; adding acids (lemon juice or vinegar) to some foods; using acceptable jars and self-sealing lids; and processing jars in a boiling-water or pressure canner for the correct period of time. Collectively, these practices remove oxygen; destroy enzymes; prevent the growth of undesirable bacteria, yeasts, and molds; and help form a high vacuum in jars. Good vacuums form tight seals, which keep liquid in and air and microorganisms out.
Many canning practices are concerned with preventing the growth of the bacterium Clostridium botulinum, which may cause botulism, a deadly form of food poisoning. Botulinum spores are found on most fresh food surfaces but, because they grow only in the absence of air, are harmless in fresh foods. Thus, most canning processing times ensure destruction of the largest expected number of heat-resistant microorganisms in canned foods, including botulinum spores. Properly sterilized canned food will be free of spoilage if lids are sealed and jars are stored below 95° F.
Further, whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity of the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods are not acidic enough to prevent the growth of these bacteria, while acid foods contain enough acid to block their growth, or destroy them more rapidly when heated. The term “pH” is a measure of acidity; the lower its value, the more acid the food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.
Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6 unless their recipes include enough lemon juice, citric acid, or vinegar to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.
Botulinum spores are very hard to destroy at boiling-water temperature; the higher the canner temperature, the more easily they are destroyed. Therefore, all low-acid foods should be sterilized at temperatures of 240° to 250° F., attainable with pressure canners operated at 10 to 15 pounds per square inch of pressure as measured by gauge (PSIG). At temperatures of 240° to 250° F., the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of the jars. The time needed to safely process low-acid foods in a boiling-water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes.
Accordingly, it is very important in the art of home canning to destroy undesirable bacteria such as Clostridium botulinum by processing the cans in a pressure canner or boiling-water canner. But using such high-pressure and high-heat devices presents other safety concerns to users, such as the handling of equipment used in the canning process. One such concern is with the handling of canning racks, which are used to submerse the jars in two main types of canners for heat processing home-canned foods; boiling-water canners and pressure canners.
For example, when handling jars in a conventional canning process, jar lifters are used to individually place and remove jars in a rack of the canner. The rack positions the one or more jars in the canner such that water substantially surrounds each jar. While the rack is removable from the canner, the jar lifters are used to place the jars in the rack and remove the jars from the rack because, typically, the rack is at least partially submersed in hot water. Thus, to avoid injury to the user from the hot water, the jar lifters provide a safe way to move the jars to and from the hot water. This process suffers from the inefficiency that only a single jar may be removed at one time, despite the fact that the rack itself is removable from the canner.